The separation between surfaces in solvent is governed by several non-specific short-and long-range forces, including the repulsive hydration and steric forces and the attractive van der Waals force. Experiments are proposed to measure and analyze these interactions between (1) several types of phospholipid and phospholipid- cholesterol bilayers and (2) bilayers containing glycolipids and glycoproteins. The hydration and steric forces, which provide the major barriers to the close approach of bilayers, will be measured as a function of separation between adjacent bilayers by x-ray diffraction analysis of multilayers compressed by known osmotic and vapor pressures. Electron density profiles will be used to determine the bilayer structure and fluid spacing for each value of applied pressure. X-ray diffraction and calorimetry experiments are designed to determine the dependence of the hydration force on the dimensions of the solvent molecule and the structure, composition, and properties of the bilayer. The hydration force will be measured as a function of lipid head group type, head group correlations (long-range order) in the plane of the bilayer, presence of membrane components such as cholesterol, glycolipids, and n-alkylresorcinol, and interfacial potential. Data from these experiments will be used to test several theoretical models, and should provide solid information on the physical basis of the hydration force. Sharp upward breaks in the pressure-spacing relation at small interbilayer separations will indicate the onset of steric hindrance. These studies will indicate the degree to which various phospholipids, glycolipids, and glycoproteins modify the magnitude and range of steric interactions between membrane surfaces. In addition, the pressure-spacing relations will be used to determine the plane of origin of the van der Waals force. The precise location of this plane and the magnitudes of the hydration and steric forces are essential parameters for calculations of interbilayer adhesion energies and for quantitative analyses of membrane fusion.